1. Field of the Invention
This invention relates to diesel engines for locomotives and the like; and, more particularly, to diesel engines whose emissions must meet Tier 0 emissions standards promulgated by the Environmental Protection Agency (EPA).
2. Background Art
In a diesel engine, fuel is directly injected into a cylinder of compressed air at a high temperature. The fuel is broken up into droplets which evaporate and mix with the air forming a combustible mixture. Products of combustion of this mixture are exhaust emissions that include hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM). To reduce the amount of pollution in the atmosphere, the EPA regulates the emission level of these various exhaust products that is acceptable. Over time, the acceptable levels of emissions have been significantly reduced.
Attainment of these standards involves consideration of a number of factors relating to engine operation. These include such things as injection pressure and injection timing, nozzle spray patterns, hydraulic flow, manifold air temperature, compression ratio, and air/fuel ratios. As will be appreciated by those skilled in the art, changes to effect reduction of one type of emission may well result in an increase in another emission component. For example, retarding fuel injection timing, which effectively reduces NOx, also affects engine performance.
It is desirable, therefore, to effect a strategy for in-cylinder combustion which satisfies the Tier 0 requirements for NOx, while at the same time maintaining an acceptable level of engine performance, including fuel consumption.